High-frequency filter



March 28, 1950 JENKS 2,501,677

HIGH-FREQUENCY FILTER Filed Sept. 24, 1943 I N V E N T O R Hearse/c A. JEN/ 8 BY@@W Patented Mar. 28, 1950 HIGH-F-REQUENCY FILTER Frederic A. Jenks, "Rockville Centre, N. Y., assignor toThe Sperry Corporation, a corporation of Delaware Application September 24, 1943, Serial No. 503,709

' (o1. ne er) 8 Claims. 1

. ,My invention relatesto high ifrequency apparratus and particularly to.arrangements for preventing undesired transmission of. high frequency energy. Theinvent'lonmill 'be .describedfor-iits preferred embodiment .as employed .for .blocking undesired passage of ultrahigh frequency energy into an ultra high frequency receiver or the like along input or output leads normally provided to carry low frequency or continuous currentenergy. (It will'be understoodhowever that this use of the invention is onlyillustrative, andnot restrictive.

'Inultra high frequency receivers of the type employed for examplein radio object detecting and locating apparatus, wherein the antenna maybe located at a different'level than the receiver, it was noted that the metal receiver chassis itself and metal parts therein served as an antenna producing confusing signal reception under many circumstances. It was discovered that considerable high frequency energy was "leaking'into the receiver through thefront panel "and especially through such non-high frequency transmission lines as power input and meteroutput leads. It was specifically in correction of this problem that .the present invention .was developed, although'the .solution .may be embodied in transmission lines or the'like wherever'high frequency suppression or similar filte action is desired.

It is therefore 'a major object of thejinvention to provide novel arrangements for preventing high frequency energy from entering a receiver or the like by way of input or output transmission lines .or conductors normally adapted to carry only"1cwfrequency orcontinuous current energy.

A further object of the invention is to pi-(k vide novel microwave'suppression filter arrangements in a transmission line.

Itjs still a furtheitoloject of the invention .to provide a high vfrequency filter embodying an :outer conductor enclosing a'relative'ly small clizameter inner conductor having one or more radially enlarged sections each presenting a peripheral surface substantially coextensive with the adjacent inner peripheral surface 'Of "the outer conductor, and wherein one 'or "both of said peripheral surfaces :are coated or "otherwise 'cov- =ered witharelativelyihigh voltage insulating-film .or covering, preferably of film thickness, which alsoserves to maintain the conductors iii-spaced relation. Specifically pursuant :to this object,

each said surface :of said 'enlargediinner conducztor section or sections, or each adjacent surface of the outer conductor, orboth, maybe provided with a permanently "bonded insulation film, as'by the process known'as anodizing.

,A further object of the invention is-to provide a novel enlarged inner conductor section for "a transmission line high frequency filter, said conductor section having an insulation-coated peripheral surface.

A further object of the invention is to provide a novel gmulti-conductor high frequency filter structure.

A further object of the invention is to provide a novel ,high frequency filter wherein a plurality of inner conductors extending through a single outer conductor are'provided with individual filter sections within the outer conductor.

Further objects of the invention will-presently appea as the description proceeds in connection with the appended claims and the annexeddrawings wherein:

Fig. ,1 is a perspective view of an ultrahigh frequency receiver equipped with my high frequency filter arrangements;

Fig.2 isan enlarged elevation partly in section illustrating a portion of the receiver panel of Fig. '1 and the mounting of my high frequency filter thereon;

Fig. 3 is a perspective view partly insection illustrating details of the block comprising the outer conductor of the filter of Figs. 1 and 2;

Fig. 4 is a fragmentary section illustrating the dinner conductor arrangements in the filter of Fig. 5 isan enlarged-section illustrating details of one filter section of Fig. A;

Fig. 6 is can end elevation of Fig.5;

Fig. 7 is an end elevational View of an optional inner conductor arrangement employable with the block ofFig. 3, wherein two conductors may ihe passe'd through a single bore in the block;

.Fig. 8 is'a perspective viewillustrating one of the inner conductors of F g. 7;

Fig. 9 isasection illustratinga variation of the .multi-conductor arrangement ofiFig. '7;

vention.

Referring toFigs. 158,116.12, aradio receiver H comprises a pluralityof elements suitably mount- ..ed on .a chassisfixe'd ito-agfront panel 42 and enclosed :by a rectangular metal box-like dust cover 13 which gisremova-hly securedto panel I2 as by screws I 4. Since details of the receiver chassis and other elements therein are non-essential to understanding the invention, none are shown or described.

On the front face of panel l2 are an antenna plug connection IS, a power input receptacle M5 for introducing continuous current power into the receiver, and a meter output receptacle H for obtainin low frequency output meter actuating energy from the receiver. The invention is primarily designed to prevent received ultra high frequency Wave energy from being transmitted to the interior of the receiver through receptacles I8 and I1 and their associated leads to the receiver elements, and the invention is mainly embodied in the microwave filter device indicated at |8 in Figs. 1 and 2.

As shown in Fig. 2, the front surface of panel l2 may be covered with a thin metallic conductor sheet l9 which is grounded to the receiver as by interior by means of a side wall aperture 23 and an aperture 24 through sheet I9 and panel 12.

Filter device It? is secured to panel l2, as by bolts 25 passin through block 2|, and the inner surface of block 2| is thus held in good conductive contact with grounded metallic sheet |9 or with the bared metal outer surface of panel l2 where sheet I9 is omitted. Apertures 23 and 24 are aligned as illustrated, and a removable cover plate 26 both physically and electrically closes the open end of recess 22. Plate 26 must have good electrical contact with block 2|, and may be integral therewith, and must have good electrical contact with sheet l9 or the metallic front surface of panel |2 so as to prevent leakage of high frequency energy into the receiver at these points.

Block 2| is suitably formed with a plurality of spaced parallel cylindrical longitudinal bores 21, each of which is adapted to contain an inner conductor assembly as illustrated in Fig. 4.. Each innner conductor assembly comprises a continuous conventional relatively small diameter wire 28 having provided thereon at regularly spaced izrgtervals a plurality of radially enlarged sections Preferably each section. 29 comprises a cylindrical annular conductive member 3| mounted upon wire 28, and secured tightly upon wire 28 as by set screws 32 which terminate Well short of the outer peripheral surface of member 3|. Preferably, also, each section 3| is made of solid aluminum or a suitable aluminum alloy and has its outer surface provided with an integrally bonded electrical insulating film or covering 33 of, aluminum oxide produced by the electrolytic process known in the art as anodizing.

Briefly, anodizing of member, 3| is accomplished by making member 3|, which is of aluminum or an aluminum alloy, the anode of an electrolytic cell containing a suitable concentration of Suifuric or chromic acid electrolyte. Activation of the cell provides in due time an aluminum oxide coating on member 3|. Upon removal from the cell, the oxide coating of member 3| is sealed, as by immersion of member3| in a boiling water or steam bath. By this step, the oxide coating is probably converted to a monohydrate of somewhat greater volume but will be referred to herein as an oxide coating or oxide coating compound, and tendency of the coating to chalk" upon prolonged exposure to air is largely prevented.

In general, substantially any known anodizing process may be employed for my purpose. For example, any of the anodizing processes described at pages 368 to 373 of Protective Coatings for Metals, by Burns and Schuh, American Chemical Society Monograph Series No. 79, published by Reinhold Publishing Corporation of New York, N. Y., may be employed.

As pointed out in the Burns and Schuh publication, conditions of the anodizing process may be varied according to the desired properties of the oxide coating 33. For my purpose, I desire the aluminum oxide compound coating to be smooth and resistant to abrasive action and to have a relatively high voltage insulation. I find thatthe process conditions best for obtaining su-ch'results include the anodization of a pure aluminum member 3|, and the hot Water sealing step. The oxide coating so produced has voltage insulation up to about 900 volts. Where member 3| comprises an aluminum alloy, the voltage insulation of the oxide coating is correspondingly lower, for example in the neighborhood of 250- 400 volts in the case of Duralumin or like alloys.

The thickness of film 33 can be controlled by suitably regulating process conditions such as current density, time, etc. For my purposes, I find a film thickness of between about .0001 inch and .00075 inch satisfactory. This film thickness must be carefully controlled for my invention since member 3| should have a smooth sliding lit with bore 21, for properly and removably supporting the inner conductor and for providing maximum distributed capacitance between the inner and outer conductors at these sections. A force fit between member 3| and bore 21 mustbe avoided to prevent scraping away of the film.

Other surfaces of member 3| than its peripheral surface are of course oxidized during anodizetion, but these are incidental and unimportant, and Fig. 5 shows only the oxide coating 33 which is concerned with the invention.

According to the above therefore, each enlarged inner conductor section 29 is provided with an integrally bonded, permanent, smooth, hard, abrasive-resistant, non-porous, uniformthickness film of insulating material. If desired. block 2| may also be made of aluminum or an aluminum alloy, and treated by anodization similarly to member 3| to provide a suitable aluminum oxide film 34 on the interior of bore 21. Film 34 preferably has the same properties as film 33. Alternatively, the invention may be carried out by provision of a suitable film 33 alone, film 34 alone, or both as illustrated in Fig. 5, the films being made of necessary thickness. Where block 2| is anodized, its rear surface is scraped to bare metal before mounting on sheet I!) or panel l2, to insure proper shielding.

Suitable co oring agents may be introduced into the anodizing process for obtaining oxide coatings of desired color, asdescribed by Burns and Schuh. This fact may be employed in the present invention by providing oxide coatings of different colors on the various conductors in the several bores 21, for easy identification purposes.

Block 2| is apertured for passage of four inner conductors 28, the internal surface of each bore 21 serving as the associated outer conductor. A

'eondu'ctor'term'inal.

"in use, the various :inner {conductors 28 'pro jectingfrom th'e uppenendo'f block' fl aresecureti to the proper receptacle terminals. .Theinner conductors 28 "projecting irom 'the lower end "of block -2l, and. wire 8! irom post 35, rare led through apertures 23 and 24 into the Iinteriorro'f the receiver-"for suitabler connections.

As shown in Figs. 2 and4, each'filter-assembly embodies three sections 29 separated "by spaces "30. Preferably 'each section 29 and-each space 5130 is "made equal in 'axial length ele ctrically to onequarter o'f thewave length of'the main high fre- *quenc-y to be suppr-essedbut'may 'be .any suitable length. "This-idea of f employing such quarter wave length spacing or other suitable length is known :an'dis not part of the present .inventionywliich invention comprises "an improvement -TOVel 'ssuch known filter arrangements.

The i-number of sections 29 -depen'ds'chiefly 'on "the" use and efficiency of :theiilter. :An accurate- 1y. made filter mayrequire only "one-section 29 for eliminating the high frequency energy. "The number of sections-2 9 isnot cr tical therefore, but

*eachadded section increases the 'filter 'fiiciency.

While the receiver .535 inuse, outer conductor block-2| isgrounded with sheet l'9or meta11pane1 I 2. Thequarter wave filter. sections on each conductor, while *permitting passage -"of low firetiv'ely' shortcircui-t' high voltage ultra high ifrerquency energy, and thereby prevent high firequency energy 'from being passedinto thereceiver except by means of antenna plug ii.

.In the iilter assen'rbly-"ofFigs. 1- 6,. each bore flsurroun'ds a single inner conductor. This-assembly may beadapted'for introducingmorethan adjacent flat faces inrflush-contact and socthat together they support theconductors withinrbore 2'las if a solid cylindrical member Was employed.

By the above arrangement each bore :21 may the madeto accommodate two inner conductors. "The {disposition and dimensions ;of.- sections 39 and $4 I ,are'the same as for sections-29cabove described.

If desired, each bore 21 may be adapted to accommodate three, four, or even more inner conductors by suitably shaping and anodizing the enlarged inner conductor sections. For example these sections would be quadrants where four conductors are accommodated.

Figs. 9 and illustrate an embodiment of the invention similar to that of Figs. '7 and 8, but

wherein separate integral inner conduct-or members are provided. As shown in Fig. 9, two inner conductors 30 and are supported within a single bore 21. As shown in Fig. 10 each semicylindrical conductor 40 is integrally formed with a semicylindrical enlarged section 46, similar in Each section 39 comprises a solid aluminum shaped:ozsectiornflI "semicylindrioal conductor? is identically tier-med with 'an enlarged integral semicylindrical :isection 41 similar to :section 23.9.

' lreierably teach iinner rconductor member is cast intone piece from aluminum 'ornan. aluminum:;al- 10y. in xoneeor: more-sections :as required,;and then both-are anodized toiprovide overs-all aluminum oxide :coatings .esirnilar 'to film 1.33. These oxide coatings: are: not'zshownin detail.in.Fig-s...9 and I0.

In use, theitwo.integral conductors are assembled within bore 121 "with their oxide .c'oatedcflat sides. inifull surface:contact,rand'withsections 46 and :41 :coextensive. .The -oxide 'coatings l insulate the inner conductors from each other andithe -otherconductors #for :low voltages as :in Figs. 7 and 8. The -arrangement of 'Figsu9 and lorenables :me;-to:.locate the two inner conductors'in .maximum :.proximity :and ithereb /avoid as much as possible undesirable high frequency effects which "mayarise'by reason of the'parallelawire 'assembly'entaile'd in'the'construction.

IJIIf desiredzof :course inner conductors 28,2. 4

the-unit. 2A:series of cylindrical.enlargedsec- *tions -25l taretzprovidedralong conductor :49 :within itube48.

Each section "5| preferably comprises ran anodized cylindrical :plug .56 "threaded on conductor 49, thezaluminumroxide filmsfil [On itspe- 'riphery fitting snugly :and smoothly within the innerrperiphery of tube .48. 'Each section :5! is .preferably'ofran axial. length equal to one-quarter (of the *operating frequency but may be :;any length desired, rand is "spaced by an a. equal :dis- :tance.

.By :employing anodized inner conductor enlargements in this filter unit, :I 'am enabled :to provide ':a more easily .zassembled arrangement which eliminates the polystyrene sleeve hitherto used tbetween the conductorsaas .well as obtaining the other 'above enumerated advantages. Terminals 52, 753 and 54, V are provided at opposite ends fdf ithe "filter unit for attachment of suitable cconductors. To maintain "the "conduc- Itors against'axial displacement in the assembly ofE'Fig. 'lll,the outer surface of sections 5| may be *wet during: assembly with: a suitable adhesive whichdriesto secure the conductors together.

"The invention is not limited to the-provision of insulatingfilms'such 'as 33, 34, 44, 53, etc.,by anodization 'of 'aluminum. It contemplates any manner 'o'f providing asubstantially permanent insulating 'film 'or coating :suitable for *the purpose. For-example sections 29-.may be' of copper "rorzany suitable conductive material, while film -33 I outer conductor, a pair of spaced inner conductors disposed within said outer conductor, a plurality of coextensive radial projections of substantially semi-circular cross-section attached to said inner conductors, the length of each of said projections and the spacing between the respective pairs of projections exceeding the inner diameter of said hollow outer conductor and each of said projections having a peripheral surface closely adjacent the coextensive inner surface of said outer conductor and each pair of coextensive projections having mated adjoining faces, and a permanently bonded film of insulating material covering each of said projections.

2. A high frequency filter comprisin a hollow outer conductor, a plurality of spaced inner conductors disposed within said outer conductor, partially cylindrical-shaped coextensive radial conductive projections on each of said inner conductors having a preferable axial length equal to one-quarter of an operating wavelength, and means insulating said inner conductors from each other.

3. A high frequency filter comprising a hollow outer conductor, inner conductors disposed therein, a plurality of conductor slugs encircling said inner conductors, said slugs extending from said inner conductors toward said outer conductor, means supporting said inner conductors within said slugs, means insulating said outer conductor from said conductor slugs, and further means insulating said conductor slugs from each other said slugs being preferably a quarter of an operating wavelength long and separated from the next slug by an equal distance.

4. A high frequency filter comprising a hollow outer conductor, inner conductors axially disposed therein, a plurality of conductor slugs encircling and affixed to said inner conductors, said slugs extending from said inner conductors toward said outer conductor, means insulating said outer conductor from said conductor slugs, and further means insulating said conductor slugs from each other said slugs having an axial length equal to the axial length between successive slugs.

5. A high-frequency filter comprising a hollow outer conductor, inner conductors axially disposed therein, a plurality of conductor slugs encircling and affixed to said inner conductors, said slugs extending from said inner conductors toward said outer conductor, means insulating said outer conductor from said conductor slugs, and further means insulating said conductor slugs from each other, said conductor slugs being substantially semi-circular in cross-section and closely diametrically abutting each other, and having axial lengths of the order of one-fourth the wavelength at the operating frequency.

6. A high frequency filter comprising a hollow outer conductor, inner conductors disposed therein, and extending throughout the length thereof a plurality of conductor slugs of circular segmental cross-sections encircling said inner conductors, said slugs extending from said inner conductors toward said outer conductor and having closely abutting diametric faces and being of the order of one-fourth wavelength long at the. operating frequency, means insulating said abutting faces from each other, means supporting said inner conductors within said slugs, and further means insulating said conductor slugs from said outer conductor.

7. A high frequency filter comprising a hollow outer conductor, an inner conductor axially disposed therein, said inner conductor being split diametrically to form separate conductors having diametrically abutted faces, a plurality of pairs of semi-cylindrical conductor slug sections, each pair having diametrically abutted faces, said split conductors being axially aligned with'said pairs of semi-circular slug sections and forming continuations thereof, means insulating each of said split conductor faces and said slugsection faces from its abutted diametric face, and further means insulating said slugsections from said outer conductor.

8. A high frequency filter embodying a multiconductor transmission line section comprising an outer conductor having a bore therethrough and a plurality of inner conductors disposed axially symmetrically of said bore, said inner conductors having radially extending enlarged diameter conductive sections spaced in seriatim and at predetermined regular intervals, and a film of insulating material between the peripheral surfaces of said sections and said outer conductor said enlarged conductor diameter sections and said predetermined regular intervals being preferably equal to a quarter of an operating wavelength long and alternating with each other.

FREDERIC A. JENKS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 758,598 Richmond Apr. 26, 1904 1,874,542 Kaul Aug. 30, 1932 1,904,162 Milliken Apr. 18, 1933 1,993,526 Rihl Mar. 5, 1935 2,044,580 Leach June 16, 1936 2,046,707 Swoboda July 7, 1936 2,088,949 Fekete Aug. 3, 1937 2,097,519 Gabriel et a1 Nov. 2, 1937 2,178,299 Dallenbach Oct. 31, 1939 2,194,543 Curtis Mar. 26, 1940 2,253,503 Bowen Aug. 26, 1941 2,296,678 Linder Sept. 22, 1942 2,342,254 Dallenbach Feb. 22, 1944 2,438,913 Hansen Apr. 6,1948 

